A structural approach to pathological crystallizations. Gout: the possible role of albumin in sodium urate crystallization.

The interactions between sodium urate monohydrate (MSU) crystals and human serum albumin (HSA) were investigated in vitro in relation to the disease of gout. It was found that HSA accelerates (by up to ten times or even more) the nucleation of MSU crystals at a pH of more than 7.5, but only to a much lesser extent (1.2 times) at pH 7.0. Protein denaturation, as well as blocking exposed carboxylate groups on the protein, substantially reduced the nucleating effect. By use of immunofluorescence, immunogold labelling and crystal morphology studies, albumin was shown to interact preferentially with the (110) faces of MSU crystals. Taking these results into consideration, a mechanism is proposed whereby albumin stabilizes MSU crystal nuclei by interaction of structured carboxylate-containing protein domains with planes of the incipient crystal exposing sodium cation layers.